I have a HD video that plays fine locally on my computer. But when uploading it to YouTube or Vimeo and then playing it there, it will have a section where the video is frozen and no sound is playing at all. Both video and sound may resume after this section, but not for all uploads I tried.
What is causing this malfunction?
In the same YouTube / Vimeo upload, the freezing was always at the same time when playing it multiple times. So it seems to be an issue with the video data, not with the playback software.
When comparing different uploads, the freezing never happened at exactly the same time, but often would last from around 5:30 (±10 s) to around 7:30 (±10 s). It seems to always be located somewhat later in the video, not right at the start.
The issue will not happen with short (~30 s) videos, even when these contain the exact clip that would freeze when inside a longer video.
YouTube greeted me one time with a message "Processing 99%. Processing is taking longer than expected. Hold tight." But when visiting the video's link, the video was already there and watchable in SD resolution (that is, processing had finished).
On one video, the frame where the freeze started was only visible properly in the upper half while its lower half had been replaced with vertical stripes, colored according to the last line of pixels shown of the upper half.
I am using the Firefox browser and have some extensions installed and enabled, including AdGuard.